spi: spidev: fix lock inversion between spi_lock and buf_lock

In the Linux kernel, the following vulnerability has been resolved:

spi: spidev: fix lock inversion between spi_lock and buf_lock

The spidev driver previously used two mutexes, spi_lock and buf_lock,

but acquired them in different orders depending on the code path:

write()/read(): buf_lock -> spi_lock

ioctl(): spi_lock -> buf_lock

This AB-BA locking pattern triggers lockdep warnings and can

cause real deadlocks:

WARNING: possible circular locking dependency detected

spidev_ioctl() -> mutex_lock(&spidev->buf_lock)

spidev_sync_write() -> mutex_lock(&spidev->spi_lock)

*** DEADLOCK ***

The issue is reproducible with a simple userspace program that

performs write() and SPI_IOC_WR_MAX_SPEED_HZ ioctl() calls from

separate threads on the same spidev file descriptor.

Fix this by simplifying the locking model and removing the lock

inversion entirely. spidev_sync() no longer performs any locking,

and all callers serialize access using spi_lock.

buf_lock is removed since its functionality is fully covered by

spi_lock, eliminating the possibility of lock ordering issues.

This removes the lock inversion and prevents deadlocks without

changing userspace ABI or behaviour.